There have been previous proposals for a transducer element to be integrally formed in a portion of a ferromagnetic shaft subject to torque about its lengthwise axis. One previous proposal is disclosed in PCT application PCT/GB00/03119 published under the number WO00/13081 is to longitudinally magnetise the shaft portion so as to create a torus of axially-directed magnetisation. Torque is sensed by detecting a tangentially or circumferentially-directed component of magnetic field, which component is torque dependent. In order to assist an understanding of the present invention, there will first be outlined an implementation of the previous proposal with reference to FIGS. 1-3c of the accompanying drawings.
FIG. 1 shows how an annulus of longitudinal magnetisation is applied to an integral portion of a shaft. The portion is to provide a transducer element and it at least is of magnetic material. In FIG. 1 a shaft 10 of magnetic material is rotated magnetic material is rotated about its axis so that a portion 12 of it is magnetised by the axially-spaced north-south poles NS of a magnet arrangement 14. This may be conveniently an electromagnetic which enables the magnetisation to be readily controlled. The magnet system may be moved about the shaft. The result of this magnetisation is to produce an annular zone of surface magnetisation 16 as shown in FIG. 2a having NS poles as indicated. It extends as an annulus about the shaft axis having the remanent magnetisation of the same polarity around the axis of the shaft and axially-directed. As indicated, the annular magnetisation tends to form a closed flux path within the shaft interior to annulus 16 so that a toroid of magnetic flux is established about the shaft axis. What is important is the magnetic field detectable exteriorly of the shaft as will be shortly explained.
The toroidal flux concept can be enhanced as is shown in FIG. 2b which shows a surface adjacent annular magnetised zone 16 within which an interior annular magnetised zone 18 of opposite polarity is established.
The two zones combine as shown to provide the torus of closed loop magnetic flux. The magnetisation is obtained by a two-step procedure. Firstly a deeper annular region of the polarity of zone 18 is formed by the magnet 14. Then the surface adjacent zone 16 is formed by reversing the magnetisation polarity of the surface adjacent region of the deeper region.
Turning to the practical utilisation of the resultant transducer element, reference is made to FIG. 3a which shows the magnetic field of zone 16 as seen at the surface of the shaft in the absence of torque. The arrow Mf indicates a fringing field which will extend generally in the axial direction between the poles of region 16 in the ambient medium, usually air.
FIG. 3b shows the effect of putting the shaft, and thus transducer element portion 12, under torque in one direction about the axis A—A of shaft 10. The longitudinal field in zone 16 is skewed as shown by the arrows (the skew is exaggerated for clarity of illustration). The external fringing field is likewise skewed or deflected as represented by magnetic vector Mf FIG. 3c). Also generated is a vector component Ms which in this embodiment extends circumferentially about the circumference of shaft 110. The component Ms is tangential to the shaft at any point, that is perpendicular to the local radius. It is the Ms component that provides the component for measuring torque by means of an appropriately oriented magnetic field sensor or group of sensors Ms is a function of torque. If the torque is in the opposite direction the direction of Ms is reversed. At zero torque, Ms has a zero value.